Both electromagnets and permanent magnets have been used to manipulate beams of charged particles. In traveling wave tubes for example, magnets have been arranged around the channel through which the beam travels to focus the stream of electrons; that is, to reduce the tendency of the electrons to repel each other and spread out. Various configurations of permanent magnets have been attempted in an effort to increase the focusing effect while minimizing the weight and volume of the resulting device. In conventional traveling wave tubes, permanent magnets are typically arranged in a sequence of alternating magnetization, either parallel to, or anti-prallel to, the direction of the electron flow. The magnets are usually annular in shape and their axes are aligned with the path of the electron beam.
Those concerned with the development of magnetic structures have continually searched for a means to maximize the strength of the magnetic field without increasing the mass of the magnetic structure.